Inkjet printing mechanisms use cartridges, often called "pens," which shoot drops of liquid colorant, referred to generally herein as "ink," onto a page. Each pen has a printhead formed with very small, pin-hole-sized nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization or firing chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a "service station" mechanism is mounted within the printer chassis so the printhead can be moved over the station for servicing and maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting," with this non-image producing waste ink being collected in a "spittoon" reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide faster drying, more waterfast printing with darker blacks and more vivid colors, pigment based inks have been developed. These pigment based inks have a higher solid content than the earlier dye based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to use plain paper. Unfortunately, the combination of small nozzles and quick drying ink leaves the printheads susceptible to clogging, not only from dried ink and minute dust particles or paper fibers, but also from the solids within the new inks themselves. Partially or completely blocked nozzles can lead to either missing or misdirected drops on the print media, either of which degrades the print quality. Besides merely forcing clogs out of the nozzles, spitting also heats the ink near the nozzles, which decreases the ink viscosity and assists in dissolving ink clogs. Spitting to clear the nozzles becomes even more important when using pigment based inks, because the higher solids content contributes to the clogging problem more than the earlier dye based inks.
The pen body may serve as an ink containment reservoir that protects the ink from evaporation and holds the ink so it does not leak or drool from the nozzles, Ink leakage is prevented using a force known as "backpressure," which is provided by the ink containment system. Desired backpressure levels may be obtained using various types of pen body designs, such as resilient bladder designs, spring-bag designs, and foam-based designs.
To maintain reliability of the inkjet printing mechanism during operation, it would be helpful to have advanced warning for an operator as to when the ink level in a cartridge is getting low. This would allow an operator to procure a fresh inkjet cartridge before the one in use is completely empty. If the cartridge is refillable, an early warning would allow an operator to replenish the ink supply before the pen is dry-fired. Dry-firing an inkjet cartridge when empty may cause permanent damage to the printhead by overheating the resistive heater elements, causing the resistors to burn out.
A variety of solutions have been proposed for monitoring the level of ink within inkjet cartridges, with many incorporating measuring devices inside the cartridge. For example, several mechanical devices have been proposed to determine when the ink supply falls below a predetermined level. One system uses a ball check valve within an ink bag to interrupt ink flow when the pen is nearly empty. Unfortunately, this system has no early warning capability and it may abruptly interrupt a printing job when a certain level of ink is reached.
Other earlier ink level monitoring systems kept a running count of the number of drops fired, which worked well until cartridges were exchanged. Unfortunately, these drop counting systems had no way of determining whether a new or a partially used cartridge was installed, so they failed to detect upcoming empty conditions for the partially used cartridges. Several more sophisticated detection systems have been devised, based upon measuring printhead temperature changes after spitting specific amounts of ink into the spittoon. These temperature monitoring systems were slow to use, and they wasted ink that could otherwise have been used for printing. Other systems have been proposed using specially designed nozzles which are more sensitive to changes in the ink reservoir backpressure than the remaining nozzles, with these backpressure changes indicating ink depletion.
In operating an inkjet printing mechanism, it would be helpful to provide feedback to a print controller, such as a printer driver residing in an on-board microprocessor and/or in the host computer, as to whether or not the printhead nozzles are firing as instructed. This information would be useful to determine whether a nozzle had become clogged and required purging or spitting to clear the blockage. This information would streamline the spitting process and conserve ink because only the clogged nozzle(s) would be spit to clear the blockage. Moreover, if damaged nozzles or heating elements could be detected, then other nozzles may be substituted in the firing scheme to compensate for the damaged nozzles. Feedback as to nozzle firing could also be used to test the electro-mechanical interconnect between a replaceable inkjet cartridge and the printing mechanism. Over time, this interconnect may be contaminated with ink, interrupting the electrical connections. When this happens, it would be desirable to notify the user to clean the interconnect.
As a manufacturing quality control check, it would also be desirable to monitor nozzle performance, for instance, to verify correct nozzle-to-nozzle alignment. It would also be helpful to check for any nozzle telecentricity, that is, any lack of perpendicularly of the orifice hole through the nozzle plate relative to the plate surface. Another important feature to monitor would be nozzle directionality, that is whether a nozzle was firing at an angle other than perpendicular to the orifice plate and/or to the media.
It would also be useful to determine from merely firing ink droplets at media, what type of media was inserted into the printing mechanism, such as plain paper, glossy high-quality paper, or transparencies. This information would then allow the printer controller to adjust the print mode to correspond to the type of media in use. One desirable energy saving would be to use only the minimum "turn-on" energy required to eject ink from each of the nozzles. Using only the minimum amount of firing energy would extend printhead life by minimizing overheating of the heaters in the printhead. This minimum firing energy operation could be accomplished by providing drop feedback to the printer controller.
In the past, some inkjet printing mechanisms have detected drops using optical means. For example, one system measured the change in drop volume for a given firing temperature by firing smaller and smaller droplets until the drops could no longer be seen by the optical detector. Unfortunately, the target drop volume has decreased in newer inkjet cartridges, for instance, some droplets are now on the order of 30 picoliters. These small droplets require precise positioning of such an optical drop detector, which is difficult to implement consistently and reliably in production printing mechanisms. Other drop detect systems addressed the nozzle-to-nozzle and the printhead-to-printhead alignment issues by printing several test patterns, from which a user then selects the best pattern or compares the test pattern to a reference pattern in the instruction manual. In these visual tagging systems, the printer controller or driver then adjusts the printing mode to an optimum level that corresponds the pattern selected by the user. Another visual system uses a tab connected to the internal spring-bag reservoir to retract the tab as the pen empties, giving the user a visual ink level indicator on the pen body. Unfortunately, these visual tagging systems required user intervention or judgment, so they were not automatic or "transparent" to the user in operation.
In multi-printhead systems, such as those carrying two, three, four or more cartridges, it would also be desirable to have an automatic method of monitoring the pen-to-pen alignment. This pen-to-pen alignment could then be used to adjust the firing sequence of the nozzles to compensate for any misalignment of the pens. Pen-to-pen misalignment may be caused by improper seating within the pen carriage, or an accumulation of tolerance variations within a specific pen body and printhead of a particular cartridge. Pen-to-pen misalignment may also be caused by an accumulation of tolerance variations within a specific printer carriage which holds the cartridges.
Thus, a need exists for a system to provide inkjet droplet information to the printing mechanism controller. This information would allow the controller to respond by adjusting droplet formation or print modes, servicing the pen, or alerting the operator of a particular condition, for instance, that an inkjet cartridge is nearly empty.